Mixers, modulators and other types of frequency translation devices (FTDs) influence the performance of the communication systems in which they are included. By measuring the characteristics of the FTDs, designers of communication systems can compensate for conversion loss, phase nonlinearity and other characteristics of the FTDs, so that performance of the systems can be optimized. However, characterizing FTDs using known techniques is difficult. A technique disclosed by Clark et al. in U.S. Pat. No. 5,937,006 relies on three transmission measurements that are performed on three pairs of interchanged FTDs to extract characteristics of a designated one of the FTDs. Interchanging the FTDs is time consuming and can introduce measurement errors due to impedance mismatches between the pairs of FTDs and nonrepeatability in the integrity of the electrical connections between the interchanged FTDs. There is a need for a method for characterizing FTDs that has low measurement error and that is also quick to perform.
A method for characterizing frequency translation devices (FTDs) constructed according to the preferred embodiment of the present invention has low measurement error and is quick to perform. In the method, a stimulus signal is applied to a first port of a frequency translation device and a drive signal is applied to a second port of the frequency translation device. A third port of the frequency translation device is coupled to an input of a filter. The frequency translation device, at the third port, provides a translated signal having a sum signal component and a difference signal component. A first, second and third reflection response to the applied stimulus signal are obtained with alternative terminations coupled to an output of the filter. The reflection responses include variations in either the sum signal component or the difference signal component as designated by the filter, where the variations depend on which of the alternative terminations is coupled to the output of the filter. As an alternative to the filter and alternative terminations, a frequency selective tuner is coupled to the third port of the FTD, providing variable impedance matches to a designated one of the signal components and providing an invariant impedance match to the signal component that is not designated. Flow graph terms for the frequency translation device and filter are extracted from the obtained reflection responses.